The present invention relates to a method and associated tooling to assist flow, consolidation, and cure of a fiber-reinforced, viscous thermoplastic resin composite by inputting vibration energy at high frequency and low amplitude displacement to supplement conventional autoclave heating, especially for high melting, non-Newtonian, pseudoplastic resins.
Aerospace thermoplastic composites are relatively difficult to process because the resins contain significant amounts of solvent and cure at relatively high temperatures often with a limited range of temperature between the boiling point of the solvent, melting point of the resin, and curing temperature of the resin. We call this temperature range the processing window with conventional autoclave processing where the prepreg laminate is enclosed within vacuum bags and heated within a pressurized oven, it is often difficult to obtain substantially fully consolidated products. Operating in the narrow processing window is difficult, but doing so is essential to evaporate the solvent, to melt the resin so that plies in the laminate will consolidate and flow, and to cure the resin by its chain extension condensation reaction. Augmenting the processing with ultrasonic vibration to supplement the conventional practice of pressing the melted material for momentum transport (xe2x80x9cflowxe2x80x9d) should improve the products while reducing the cure cycle. Therefore, the process of the present invention saves time and reduces waste or rework. Since the resins cost over $100 per pound and the manufacturing process is relatively slow and labor intensive, the present process promises a significant economic benefit.
In U.S. Pat. No. 4,288,398, Lemelson described alternative methods for controlling the internal structure of molded or extruded plastics or metals. Lemelson suggested using ultrasound alone or in combination with other forms of energy to orient the grain or crystalline structure. Lemelson introduced ultrasound to the melted material during its consolidation to control the internal structure. The process of the present invention uses ultrasound to assist momentum transport after heating the resin to its softening or melting temperature and during the pressure application for resin flow phase of its consolidation.
Thermoplastic resins that cure to composites usable at high operating temperature generally require high processing temperatures. For a resin-fiber composite system capable of operating at 425xc2x0 F. or higher, the resin must have a glass transition temperature (Tg) of 525xc2x0 F. after equilibration with the operating environment and an xe2x80x9cas processedxe2x80x9d Tg approaching 600xc2x0 F. A resin with a high Tg will also have a high melting (Tm) or softening (Ts) temperature. The temperature differential between the Tg and the Ts is established by the molecular weight distribution and usually is on the order of 200xc2x0 F. In addition, the viscosity of such a high melting resin above the melt or softening temperature will likely be greater than 106 Paxc2x7sec. Therefore, consolidating acceptable quality laminates using these resins requires high pressures and temperatures.
To date, only small parts that will fit within the platens of a press could be fabricated using extremely high Tg resins. Processing in this manner requires matched tooling and has been limited to high value parts such as engine components.
Attempts to consolidate large planform area parts (such as exterior skin panels for composite aircraft) in an autoclave have been unsuccessful. The laminates exhibited extensive porosity and suffered from microcracking because of the volatiles and by-product gases generated during the condensation reaction of the resin when it cured. High viscosity of these high melting resins inhibited momentum transport and resin flow during the pressurized portion of the autoclave cycle. While processing might be possible at even higher temperatures and pressures, conventional autoclaves are not designed for the increased pressures. Replacing conventional autoclaves to allow higher pressure operation is too expensive to justify using the high melting resins available today for today""s applications.
In the present invention, piezoelectric transducers apply vibration at high frequency (in excess of 105 Hz) and low displacement (i.e., xe2x89xa61 xcexcm) to a prepregged part on a layup mandrel to advance the consolidation of composites containing high viscosity resins that have high melting temperatures, high glass transition temperatures, and exhibit pseudoplastic rheology. The resin flow is increased by subjecting the parts to be processed to high frequency vibration (high shear rates), which usually exceeds 106 Hz. The vibration causes the melted resin to flow into voids in the part and incrementally heats the resin to assist in its consolidation. To avoid distortion of the individual fibers, yarns, or plies the displacement must be limited. If individual fibers translate relative to each other (i.e., move independently), then the displacement should not exceed 5-10 xcexcm, which is comparable to the fiber diameter. If the yarns or individual plies vibrate together, then the displacement can be increased to 0.005-0.01 inches. The viscosity versus shearing rate behavior will be pseudoplastic i.e. shear thinning, to provide the desired effect on the viscous resin. At these sufficiently high shear rates (one reference states above 106 Hz), the viscosity of the pseudoplastic resin will revert to Newtonian behavior which will facilitate flow of the resin at pressures that are achievable with conventional autoclaves.
Processing of a laminate containing a high viscosity resin includes (1) laying up prepreg plies by hand or with fiber placement machines in a desired pattern on a layup mandrel, (2) applying a suitable vacuum bag around the plies, and (3) placing the bag in either a specially equipped oven or an autoclave for consolidation. The surrounding atmosphere is pressurized to apply pressure to the vacuum bagged part. The bagging might include a diaphragm chamber that can be pressurized or the entire autoclave can be pressurized, or both approaches can be used. In the present invention, the oven or autoclave is equipped to provide high frequency, low displacement vibration to the part during the heating and pressure application phase of the consolidation cycle to promote resin flow by converting the rheology from pseudoplastic to Newtonian.
Our approach involves installing piezoelectric transducers in the layup mandrel and providing electrical power through an appropriate connection after positioning the tool in a conventional oven or autoclave. The propagation of acoustic waves (i.e., the pressure wave imparted by the transducer at ultrasonic frequencies) through the part involves periodic fluctuations in pressure and displacement. At a high power density, the pressure amplitude can reach or exceed 1000 psi at displacements less than 1 xcexcm (10xe2x88x926 m). The ultrasonic vibration aids mass transport of volatiles out of the resin by increasing the probability of nucleation.
Ultrasonic vibration may also aid in the compaction of dry thermoplastic prepreg tape that is being laid with a hot-head automatic tape layer. The vibration may aid-in the local compaction of the material under the tape head when the material is laid down to reduce the pressure requirement during the laying process. A reduced pressure may allow the use of a conformal rubber application head rather than a conventional rigid head.
We can also introduce the ultrasonic energy into a part by placing a cover panel or blanket containing the piezoelectric transducers over the part surface to achieve the necessary conduction path.